The COMMAREA

The use of the COMMAREA option on the RETURN command is the principal example of a safe programming technique that you can use to pass data between successive transactions in a CICS® pseudoconversational transaction.

CICS treats the COMMAREA as a special form of user storage, even though it is CICS that issues the GETMAIN and FREEMAIN requests for the storage, and not the application program.

CICS ensures that the contents of the COMMAREA specified on a RETURN command are always made available to the first program in the next transaction. This is true even when the sending and receiving transactions execute in different target regions. In a pseudoconversation, regardless of the target region to which a dynamic routing program chooses to route the next transaction, CICS ensures the COMMAREA specified on the previous RETURN command is made available in the target region. This is illustrated in Figure 1.

Some general characteristics of a COMMAREA are:
  • Processor overhead is low.
  • It is not recoverable.
  • The length of a COMMAREA on a RETURN command can vary from transaction to transaction, up to a theoretical upper limit of 32 763 bytes. (However to be safe, you should not exceed 24 KB (1 KB = 1024 bytes), as recommended in the Application Programming Reference manual, because of a number of factors that can reduce the limit from the theoretical maximum.)
  • CICS holds a COMMAREA in CICS main storage until the terminal user responds with the next transaction. This can be an important consideration if you are using large COMMAREAs, because the number of COMMAREAs held by CICS relates to terminal usage, and not to the maximum number of tasks in a region at any one time.
  • A COMMAREA is available only to the first program in the next transaction, unless that program explicitly passes the data to another program or a succeeding transaction.
Figure 1. The use of a COMMAREA by a pseudoconversation in a dynamic transaction routing environment
TOR 1 routes transaction TRN1 to AOR1. TRN1 executes in AOR1 and returns with COMMAREA (data) and TRANSID (TRN2). TOR1 then routes TRN2 to AOR2 where it executes with access to the COMMAREA data passed by TRN1.

The COMMAREA used in a pseudoconversational transaction, as shown in Figure 1, can be passed from transaction to transaction across a CICSplex, and, provided the COMMAREA contains only data and not addresses of storage areas, no inter-transaction affinity is created.